This proposal is designed to develop animal models of transplantation of myeloid marrow tissue which are independent of destruction of the host's myeloid and immune systems. The new world primate, marmoset, virtually always conceives dizygotic twins. Such twins are naturally chimeric for hematopoietic tissue by virtue of common placental circulation. They exhibit no evidence of immunologic incompatibility and thus represent a fairly pure model for attempting to influence the degree of chimerism. During the time required for developing a marmoset colony, murine studies of syngeneic and allogeneic chimerism will be carried out. It is anticipated that the murine model will have predictive value for the most efficient means of altering chimerism in the marmoset.